In recent years, research and development of hybrid vehicles (HEV), electric vehicles (EV) and fuel cell vehicles are being carried out and part of these vehicles have been manufactured and sold. These types of vehicles are called electric-powered vehicles and receiving attention in view of increased environmental concern and high fuel efficiency. In the electric-powered vehicles, the use of chargeable-dischargeable power supply systems is essential. As for such power supply systems, electric devices including secondary batteries such as lithium ion secondary batteries and nickel hydride secondary batteries or electric double-layer capacitors, are generally used. Among these, lithium ion secondary batteries are particularly suitable for used in the electric-powered vehicles due to high energy density and high resistance to repeated charge and discharge.
For example, a lithium ion secondary battery applied to an electric-powered vehicle is required to have higher output performance and higher capacity. In order to achieve higher output performance and higher capacity, an increase in size and an increase in the number of layers may be considered. However, these means may be accompanied by entrance of foreign substances into separators during the manufacture of the lithium ion secondary battery. The reason of the entrance of the foreign substances is that the separators used in the lithium ion secondary battery generally include porous substrates for conducting ions therethrough. Namely, the porous substrates have a large number of pores due to the porous structure thereof so that the foreign substances easily enter through the pores. Examples of the foreign substances include dendrites and metal pieces derived from metal foil produced during the manufacturing process. In view of the safety aspect, the entrance of such foreign substances into the separators is required to be prevented because the entrance of the foreign substances may cause a short circuit.
In order to solve the above-mentioned problem, research and development of ceramic separators are being carried out in recent years. A ceramic separator has a constitution in which a ceramic layer containing ceramic particles and a binder is formed on a surface of a porous substrate. The ceramic separator provided with the ceramic layer thereon can prevent foreign substances from entering through pores of the porous substrate.
Japanese Unexamined Patent Application Publication No. 2010-055942 discloses a separator for a non-aqueous secondary battery in which a heat-resistant porous layer containing inorganic fillers (ceramic particles) is stacked on one surface or both surfaces of thermoplastic resin having a microporous film (a porous substrate).